A turbocharger used for improving fuel efficiency of a vehicle compresses combustion air by use of a compressor and supplies the compressed air to an internal-combustion engine. The compressor is driven by rotating a turbine coaxial with the compressor by use of exhaust gas energy in the internal-combustion engine.
In such a turbocharger, the compressor does not sufficiently compress the combustion air at startup of the vehicle or during running of the vehicle in a low speed range of the internal-combustion engine. As a result, a boost pressure of the combustion air supplied to the internal-combustion engine tends to be insufficient. Such an insufficient boost pressure of the combustion air is attributable to a shortage of exhaust gas energy to rotate the turbine and the compressor, the shortage resulting from a small amount of exhaust gas at startup of the vehicle or during running of the vehicle in the low speed range of the internal-combustion engine.
In order to solve the above problem, there has been proposed a technique of eliminating the insufficiency of the boost pressure of the combustion air described above by employing an electrically assisted method. This electrically assisted method is for supplementing compressor driving power by an amount equivalent to a shortfall in exhaust gas energy by driving a rotary motor directly connected to a rotating shaft connecting a turbine to the compressor. The above technique is disclosed in a brochure of Japanese Patent No. 3203869 (Patent Document 1), for example. The rotary motor of this kind is driven by a drive circuit, which supplies power by converting a direct current power supply from a battery into an alternating current, or a driver (driver module) of the drive circuit.
Since a large current that meets the rating of the rotary motor flows through the drive circuit described above, circuit elements in the drive circuit generate a considerable amount of heat. Moreover, since the drive circuit and the driver thereof are generally disposed in a space such as an engine room close to the turbocharger, a temperature around the circuit and the driver is increased by heat emitted from the engine. Consequently, in order to protect the drive circuit and the driver from the heat, it is important to monitor temperatures of the drive circuit and the driver themselves as well as their ambient temperature and to manage operations of the drive circuit and the driver according to the temperatures.
Hence, there have heretofore been taken measures including: individually monitoring the temperatures of the drive circuit and the driver themselves and the temperature in the space where the drive circuit and the driver are disposed; and stopping the drive of the rotary motor when any one of the temperatures reaches an upper limit temperature that is considered to influence the operations of the drive circuit and the driver.
Such conventional control for the drive of the rotary motor of the turbocharger leads to the following situation. Specifically, for example, once the temperature in the space where the drive circuit and the driver are disposed reaches the upper limit temperature, the drive of the rotary motor is stopped. Then, even if the temperatures of the drive circuit and the driver are below the upper limit temperature, the drive of the rotary motor cannot be restarted unless the temperature in the space where the drive circuit and the driver are disposed drops below the upper limit temperature. Such conventional control leads to overprotection of the drive circuit and the driver and could be a factor that inhibits the rotary motor from sufficiently exerting its essential function to electrically assist the turbocharger.
The present invention was made in consideration of the foregoing circumstances. It is an object of the present invention to provide a control device for a drive unit of a rotary motor for an electrically assisted turbocharger, the control device being capable of providing electric assistance of the turbocharger by drive of the rotary motor and achieving thermal protection of the drive unit in a well balanced manner in the case where the rotary motor for the electrically assisted turbocharger is driven by the drive unit including the drive circuit and the driver.